Pacific ENSO Update

4th Quarter 1998 - Vol.4 No.4

CURRENT CONDITIONS

The very strong 1997 El Niño event peaked around January 1998 and phased out in the
May to June time frame of 1998. Around this time, sea surface temperatures (SSTs)
returned to near normal values in all equatorial Pacific regions near the west coast of
South America. By July, the equatorial SSTs in the central Pacific had become more than
1øC (1.8øF) cooler than normal, indicating that weak La Niña (cold episode) conditions
had developed. This rapid cooling suggested that a strong La Niña could develop, but the
cooling slowed and has since hovered around the 1øC cooler than normal value. The
equatorial cold tongue associated with the La Niña now extends from the west coast of
South America westward to 160ø East. Some climate models are predicting strong La
Niño conditions by early 1999, while others suggest that the La Niña will remain weak
until mid-1999. With stronger than normal easterly low level equatorial winds and cooler
than normal sub-surface SSTs (a shallower than normal thermocline) across the eastern
and central equatorial Pacific, we expect some strengthening of the La Niña to a moderate
event, peaking in January 1999 and phasing out by June.
Thus, SSTs point toward a return to near normal rainfall and tropical cyclone
activity in late 1998 for most of Micronesia, but drier than normal winter and early
spring rainfall conditions are espected to prevail in the Marshall Islands and in
near-equatorial regions.

After reaching its lowest point in March 1998, the Southern Oscillation Index (SOI)
began to rise and became positive in May. It has hovered slightly above +1 since July. It
may rise to near +2.0 by March 1999, then begin to fall to normal values (near zero) by
September. This behavior of the SOI supports moderate La Niña conditions through the
northern hemisphere spring.
Thus, the behavior of the SOI points to a return to near normal rainfall and tropical
cyclone activity for most of Micronesia, but less than normal rainfall will prevail
during winter and early spring in the Marshall Islands and in near-equatorial
regions.

Sea level heights should rise slightly in the western Pacific as water temperatures warm to
deeper levels and increase the volume of basin water. Sea levels should be lower than
normal in the central and eastern equatorial Pacific where La Niña waters are colder than
normal and water volume is reduced.

In the western North Pacific, tropical cyclone activity during 1998 was the lowest on
record. While we correctly predicted that most tropical cyclone activity would be north
and west of Micronesia , and would be below normal, we did not expect such a low
number of tropical storms and typhoons. The first tropical storm did not occur until July
8 (Tropical Storm Nicole) and the first typhoon did not occur until August 3 (Typhoon
Otto). And, the only three storms to affect Micronesia were weak at the time -- Tropical
Storm Zeb, which moved 37 nm north of Yap on October 10; Tropical Storm Babs,
which also moved north of Yap on October 15; and, midget Tropical Storm Alex, which
hit Rota in the CNMI on October 11. Tropical cyclones can still affect the western half of
Micronesia until about mid-December. Any late season tropical cyclones are expected to
intensify west of the Mariana Islands and most likely hit the Philippines.

In the Southwest Pacific, 10 tropical cyclones developed east of the international date line
from October 1997 into May 1998 (typical of strong El Niño activity), with eight
affecting French Polynesia. In addition, an unexpectedly large number of cyclones (7)
also developed west of the date line, although none affected eastern Australia. Few, if
any, tropical cyclones of severe tropical storm or hurricane intensity are expected east of
the date line until the next El Niño event.

Special Section:

The Pacific ENSO Applications Center and the Water and Environmental Research Insitute

The Pacific ENSO Applications Center and the 1997-1998 El Niño

In February and March 1997, the coupled atmosphere-ocean models that PEAC staff has consulted on a
regular basis indicated the development of an El Niño warm event. By May, it was clear that an ENSO
warm event was developing very quickly, and we alerted our clients through the Pacific ENSO Update.
Chip Guard from Water and Energy Research Institute (WERI) at the University of Guam produced
rainfall forecasts in terms of percent of normal rainfall for three-monthly seasons and provided those to the
Guam and the CNMI governments. Other governments then asked for quantitative forecasts, which Chip
produced. PEAC issued its first quantitative rainfall forecasts in the Pacific ENSO Update in October
1997.

In September 1997, Chip Guard and WERI staff began briefings for government officials in the American
Flag Pacific Islands on the impending drought. Chip reviewed the rainfall and typhoon forecasts and
suggested that governments begin preparing to respond immediately. In Yap, a drought task force was in
place before Chip arrived. It had been organized in response to the forecasts issued in the Pacific ENSO
Update. Chip reinforced the message already being conveyed by the public information program that the
task force had developed. The task forces in the RMI, the FSM, Palau, and Guam developed mitigation
plans. They mounted public information campaigns to inform the public about what to expect from the El
Niño, to explain measures that could be taken to conserve water and prevent outbreaks of diseases, and to
warn of the increased wildfire risk and actions to reduce the risks. In Pohnpei State, a video was produced
and aired on the public television station four times a day from early January through May. Hotlines were
set up, brochures were developed, public service announcements were made on local radio and television
stations, and presentations on El Niño and the drought were done in schools.

The Water and Environmental Research Institute of the
Western Pacific -- University of Guam

The Water and Environmental Research Institute (WERI) at the University of Guam
conducts research in hydrology, meteorology, climatology, water quality, hydrogeology,
and other related areas. The Institute offers a BS in Pre-Engineering and an MS in
Environmental Sciences. The Institute has a faculty of eight, which includes six
professors and two research associates. WERI is one of fifty-three US land grant water
institutes, but is unique in that it is not only the water institute for Guam, but is also the
water institute for the Federated States of Micronesia (FSM) and the Commonwealth of
the Northern Mariana Islands (CNMI). Dr. H. Galt Siegrist is the Director.

As a water institute, WERI conducts annual fresh water-related research and training
for the governments of Guam, the FSM, and the CNMI. This may entail ground water
modeling, surface water resources management, water quality assessment, erosion
studies, hydraulics, hydrologic engineering, and meteorology/climatology. The
meteorology and climatology research at WERI is both basic and applied, and includes
general tropical meteorology, tropical cyclone motion and structure change, monsoon
dynamics, climate variability, climate change, and meteorological satellite techniques and
interpretation.

LOCAL VARIABILITY SUMMARIES:

Hawaii:
No Flash Flood Warnings were issued for the state in November. Urban and Small Stream
Flood Advisories were issued for portions of windward Oahu (10 Nov), and for portions of the
windward districts on the island of Hawaii (17 and 18 Nov) and the island of Maui (17 Nov).

Rainfall amounts were a mixed bag throughout the state. Individuals in some areas may feel like
the enhanced rainfall expected during a La Niña episode is here. Others, especially in the
leeward areas of the islands of Maui and Hawaii, continue to see below normal rainfall.

Large scale trade winds continued to play a dominant role in the weather pattern over the
Hawaiian Islands throughout the month. These trade winds were light during the first week of
November. However, an upper level trough to the west of the state produced relatively unstable
conditions and enhanced shower activity over the islands. Normally, the month of November
marks the start of the cool season with the island chain normally experiencing the passage of
several cold fronts or shear lines. Subtropical cyclones, or kona storms, can also develop west of
the islands to produce prolonged periods of heavy rain. During November 1998, only one shear
line (the remnant of a cold front) was able to reach the island chain. A band of showers ahead of
the shear line produced heavy rain for a brief period along the northern sections of windward
Oahu. The shear line stalled over the island of Kauai and the Kauai Channel on 11 November
before lifting out toward the northeast. Fragments of this shear line and subsequent shear lines
embedded within the trade flow hit the islands from the east northeast over the period from 12
November to 20 November. This caused enhanced trade showers, mainly over the windward
sections of the islands. Upper level troughs helped enhance the shower activity, especially over
the islands of Maui and Hawaii during this period.

For a more complete summary, and the county by county wrap-ups, please see the
November Precipication Summary from the National Weather
Service Honolulu Weather Forecast Office.

American Samoa: After several months of drought conditions, we are expecting
near normal rainfall to return to the Samoa region. As anticipated in the last Pacific
ENSO Update, rainfall for July through October of 1998 was well below normal, but
highly variable. At Pago Pago, rainfall values for July (1.42 inches) and August (1.59)
were a very low 23% of normal. Amounts began to pick up in September and October
with 4.30 inches (64%) and 6.31 inches (58%), respectively. July and August rainfall for
the Samoa region came exclusively from showers embedded in enhanced trade wind flow
or from the occasional passage of weak, decaying cold fronts (shear lines). Our last
Pacific ENSO Update stated: "Dry conditions are expected to continue into September."
and "Wet weather is expected to return in October." This occurred, and we expect
rainfall amounts to reach near normal or slightly above normal values from November
through February as the South Pacific Convergence Zone (SPCZ) is anchored over or
near the islands. With our prediction of a moderate instead of a strong La Niña event, we
don't expect the next dry season in Samoa to be affected too severely. Tropical cyclone
activity in the southern hemisphere will move west of the international date line, reducing
the threat to the Samoa region.

The rainfall predictions for American Samoa and surrounding islands are:

Guam/CNMI: After a near record drought for Guam and the CNMI, we expect
rains to return to near normal values. 1998 could well be the driest year on record
for the Mariana Islands. Rainfall began to increase in June on Guam and in July in the
CNMI as the result of heavier rain events associated with upper level disturbances in the
Tropical Upper Tropospheric Trough. For July and August, rainfall at the Guam
International Airport (GIA) was 5.30 inches (50%) and 4.44 inches (32%), and that at
Andersen Air force Base (AAFB) was 4.28 inches (40%) and 5.26 inches (39%). July and
August rainfall at Saipan International Airport (SIA) was 5.90 inches (74%) and 5.46
inches (47%). At Capitol Hill, amounts were 8.50 inches (94%) and 8.72 inches (70%),
respectively.

In September and October, rainfall at GIA increased to 16.44 (122%) and 9.27 (77%),
respectively, while that at AAFB increased to 16.69 (125%) and 8.02 inches (62%). At
SIA, the respective values for September and October were 4.89 (44%) and 4.68 inches
(43%). Capitol Hill was wetter with 8.65 inches (69%) and about 8.50 inches (71%),
respectively. Tinian rainfall was similar to that on Saipan, with values of 6-7 inches
(depending on location) in August, September, and October. The NASA rain gage
network located at the Rota Resort and Country Club on Rota showed August,
Sepetember and October amounts of 4.87 inches (36%), 23.84 inches (179%) and about
5.72 inches (44%). The Sabana area was probably somewhat wetter than the northern
Golf Resort area, but not significantly so.

Only one monsoon episode developed across the Mariana Islands during the entire year,
lasting a few days in September. Also in September, rainfall associated with disturbances
induced by upper level low pressure systems in the Tropical Upper Tropospheric Trough
(TUTT) shifted from a position north of the Mariana Islands to near 10ø North latitude.
Most of the rain by-passed the Marianas, but Guam and Rota received more of the spotty
rainfall than did Saipan and Tinian.

Tropical cyclone activity could affect the Mariana Islands until mid-December 1998. The
threat will resume in April and May 1999 and again from September to mid-December
1999.

Our rainfall predictions through December 1999 for Guam/Rota and Saipan/Tinian are:

Federated States of Micronesia:
We anticipate normal to somewhat wetter than normal conditions for most of the
north-western FSM. Most of the eastern FSM will be drier than normal, and near-
equatorial regions will be much drier than normal through the winter and early
spring of 1999. Rainfall over most of the eastern and southern FSM was below normal
for the 4-month period July-October, largely as a result of enhanced equatorial easterly
winds associated with the current weak La Niña event. These winds also brought very
dry conditions to Nauru and the more equatorial atolls of Pohnpei State. Rainfall in
northern Yap State was above normal.

Yap State: July, August, September, and October rainfall at the Yap Airport was 11.99
inches (82%), 12.03 inches (79%), 18.13 inches (134%), and 16.11 inches (135%). Both
Ulithi and Woleai Atolls had about 20-25% less rainfall than Yap Island. In September,
rainfall associated with disturbances induced by upper level low pressure systems in the
Tropical Upper Tropospheric Trough (TUTT) moved rapidly southward from a positon
along 20ø North latitude of the Mariana Islands to a position near 10ø North latitude and
130ø-140ø East longitude. These disturbances stagnated over Yap and Palau, bringing
heavier than anticipated rainfall, despite the fact that monsoon activity we expected in
late summer and early fall did not materialize. Tropical Storms Zeb and Babs did drop
several inches of rain on Yap in the first half of October. Yap State islands could be
subjected to tropical storms and typhoons until mid-December. This threat will resume in
May 1999 and again from September to December 1999.

Chuuk State: July, August, September, and October rainfall at Weno Island was 5.43
inches (45%), 12.89 inches (89%), 6.49 inches (56%), and 12.62 inches (94%),
respectively. At Lukunoch, July and August rainfall averaged 7.40 inches (65%) and
September and October rainfall averaged 9.26 inches (75%). Conditions were drier at
Polowat where July and August rainfall averaged 5.83 inches (41%) and September and
October rainfall averaged 6.88 inches (55%). Although tropical cyclone activity has been
much reduced in 1998, Chuuk State islands could be subjected to a tropical storm or
typhoon until mid-December.

Pohnpei State: July and August rainfall for Kolonia, Pohnpei were 8.53 inches (46%)
and 14.93 inches (90%), respectively. September and October values were 10.75 inches
(67%) and 14.81 inches (89%). July and August rainfall amounts for Pingalap averaged
11.83 inches (63%), while those in September and October averaged 15.77 inches (97%).
Nukuoro received an average of only 6.04 inches (35%) in July and August and an
average of 6.89 inches (46%) in September and October. Most of Pohnpei State's rainfall
came from upper level disturbances initiated by the Tropical Upper Tropospheric Trough.
In Pohnpei, the mountains also played an important role in producing rain-bearing clouds.
Nukuoro rainfall was diminished by the stronger than normal easterly equatorial winds
associated with the ongoing weak La Niña event. Pohnpei State islands will have a small
threat from tropical cyclones from October to December 1999.

Islands very close to the equator are expected to remain considerably drier than normal as a result of the
ongoing La Niña event. This will include such islands as Kapingamarangi, Nauru, Banaba (Ocean Island),
and Tarawa.

Kosrae State: July and August rainfall for Kosrae Airport was 12.20 inches (72%) and
13.56 inches (82%). Rainfall amounts decreased substantially in September (6.64 inches
-- 39%) and October (7.75 inches -- 48%) due to increased equatorial easterly winds as a
result of the current La Niña event. Rainfall at Tafunsak averaged 60% of normal for the
4-month period and that at Utwa averaged 68% of normal during the period. Rainfall is
expected to remain below normal through March. By June, rainfall is expected to rise to
near normal values as the La Niña phases out. Kosrae has a low risk of tropical cyclone
activity, especially in non-El Niño years.

Palau: Palau can expect near normal to above normal rainfall for the next year.
Rainfall at Koror in July and August was 7.52 inches (42%) and 10.16 inches (68%). In
September and October values rose to 10.09 inches (85%) and 19.05 inches (137%), as
upper level low pressure areas associated with the Tropical Upper Tropospheric Trough
(TUTT) brought disturbances that stagnated over the Palau area. This resulted in above
normal rainfall, despite the lack of an active monsoon over the region. However, farther
south in Peleliu, conditions were drier. July-September rainfall there averaged a little
over 7 inches or slightly less than 50% of normal. In October, rainfall at Peleliu rose to
14.98 inches (108%) with some effects of the TUTT-induced rains. Rainfall for Koror
should be slightly above normal until January, then near normal thereafter. More
southerly islands should be slightly drier than normal until March, then near normal
thereafter. Palau could experience a tropical cyclone from now until mid-December and
in the fall of 1999.

Marshall Islands: Despite a relatively wet summer and fall in the Marshall Islands,
the development of a moderate La Niña is expected to reduce winter and early
spring rains. At Majuro (representative of southern atolls), rainfall for July and August
was 16.29 inches (125%) and 12.05 inches (105%). Rainfall was more variable in
September and October with the respective months experiencing 9.80 inches (79%) and
19.45 inches (141%). Kwajalein (representative of northern atolls) had 12.63 inches
(121%) and 11.18 inches (111%) in July and August. September and October rainfall
was also more variable at Kwajalein with 9.04 inches (76%) and 14.69 inches (123%).
Enhanced easterly winds associated with the anticipated moderate La Niña is expected to
create drier than normal conditions in the Marshalls from December to about April 1999.

Tropical cyclone activity will not be a threat to the Marshall Islands until at least
September 1999.

We anticipate the following rainfall amounts for the Marshall Islands:

ACKNOWLEDGEMENTS and FURTHER INFORMATION

The information contained in the LOCAL VARIABILITY SUMMARIES section
and elsewhere in this issue of the Pacific ENSO Update has been
drawn from many sources. Further information may be obtained by contacting
your local National Weather Service office, or the individuals and
institutions listed below:

NOAA National Weather Service - National Centers for Environmental
Prediction (NCEP) - CLIMATE PREDICTION CENTER (CPC):World Weather
Building, Washington D.C. 20233.Contact CPC at 301-763-8155 for more
information on the ENSO Advisory, the Long-Lead Outlook for the Hawaiian
Islands, and other publications discussed in this bulletin.

University of Guam (UOG) WATER AND ENERGY RESEARCH INSTITUTE (WERI):
Lower campus, University of Guam UOG Station, Mangilao, Guam
96923 Contact C. Guard or M. Lander at (671)735-2685 for more info on
tropical cyclones and climate in the Pacific Islands.

University of Hawaii (UH) School of Ocean and Earth Science and
Technology (SOEST) DEPARTMENT OF METEOROLOGY: HIG #331, 2525
Correa Road, Honolulu, Hawaii 96822 Contact Dr. T. Schroeder at
808-956-7476 for more information on hurricanes and climate in Hawaii.

Publication of the
Pacific ENSO Update is funded in part by Grant Number
NA46GP0410 from the National Oceanic and Atmospheric Administration
(NOAA) Office of Global Programs. The views expressed herein are
those of the author(s) and do not necessarily reflect the views of
NOAA or any of its sub-agencies.